Shallow Plumbing Geometry of a Monogenetic Volcano, Lunar Crater Volcanic Field, Nevada
Abstract
The Lunar Crater Volcanic Field is a 90 km long, 20 km wide field of monogenetic volcanoes such as scoria cones, spatter ramparts, and maars. The most recent eruption was 38 ka indicating it is essentially an active volcanic field. Although scoria cones are relatively small, they can generate natural hazards similar to composite volcanoes, such as ballistic bombs, lava flows, and the disruption of air traffic due to tephra dispersal. Physical controls on eruptions must be identified and studied in order to develop better hazard assessments. The geometry of a scoria cone's shallow plumbing system plays a major role in determining the eruption characteristics. Numerical eruption models and hazard assessments assume simple geometric shapes such as straight-sided or flaring cylinders to represent the shape of the near-surface conduit; little field data exist to support these assumed geometries. A specific vent in the southern area of the volcanic field was selected because of its excellent erosional exposure. Field mapping and measurements were made of proximal pyroclastic deposits, intrusions including the feeder dike, and the eruption conduit. Orientated samples of clastogenic lava flows and magmatic intrusions were gathered and thin sectioned to determine magma flow vectors. The eruption began along a ~295 meter, non-linear fissure in areas now preserved as steep knobs. The fissure eruption created a complex spatter rampart that was later buried as activity focused to a single vent. The feeder dike is intermittently exposed along strike for 1 kilometer, and ranges from 1.5-2 meters wide, and in most cases sub-vertical with the exception of two small sub-horizontal sills ~20 meters long. Erosion has exposed a cross section of the eruption conduit up to 23.5 meters below paleosurface, revealing a distinct flare 15 meters below the paleo-surface and a maximum surface width of 18.3 meters. Felsic streaks, sourced from the rhyolitic country rock, along with crystal fabrics observed in oriented thin sections suggest magma flow toward the surface and slightly southward. The majority of proximal pyroclastic deposits around the vent are agglomerate with black and red scoria and dense basalt that is mainly of clastogenic origin. Within the vent's pyroclastic deposits are irregular magma intrusions, roughly linear, that are sourced near the central conduit. One particular intrusion transitions as it moves down slope from vertical contacts with the surrounding deposits to sub-horizontal. ~140 m from the source the intrusion dimensions become increasingly more irregular and finally transitions to a mass of dense basalt. Mapping indicates the irregular intrusion is the source of the extensive lava field to the west of the vent area.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V21B2717H
- Keywords:
-
- 8486 VOLCANOLOGY Field relationships;
- 8404 VOLCANOLOGY Volcanoclastic deposits;
- 8400 VOLCANOLOGY;
- 8434 VOLCANOLOGY Magma migration and fragmentation